Optical communication systems are known in which optical signals carrying data are transmitted from a first node to a second or receive node over an optical fiber. At the receive node, the optical signals are converted into corresponding electrical signals, which are then further processed. The optical signals may be both wavelength division multiplexed, in which optical signals having different wavelengths are combined onto an optical fiber, as well as polarization multiplexed, in which optical signals having different polarizations (e.g., transverse electric, TE, and transverse magnetic, TM) are combined onto the fiber. In addition, in one example, the optical signals may be phase modulated to carry the data.
Various techniques are known for detecting or sensing the data carried by an optical signal. In one such technique, coherent detection, a light source or laser, also referred to as a local oscillator, is provided at the receive node. Incoming light of the received optical signal, which, if polarization multiplexed, may be split by a polarization beam splitter (PBS) into two orthogonal signals, having the TE and TM polarizations, respectively. Each signal output from the PBS is combined with the light output from the local oscillator and may be passed through a 90-deg optical hybrid circuit. The optical hybrid circuit, in turn, outputs further optical signals to four pairs of photodiodes or balanced photodetectors, which, in turn, generate corresponding electrical signals.
The electrical signals, which are typically in analog form, are next supplied to an analog-to-digital converter (ADC) circuit, which operates at a sampling rate to generate a series of digital samples at periodic time intervals. Each sample includes a plurality of bits. The samples may then be supplied to a digital signal processor (DSP), which processes the samples to extract the data carried by the optical signals.
The optical signals may be subject to various impairments including chromatic dispersion (CD). CD is due to various frequency components in each signal traveling at different velocities. CD, however, can be compensated or corrected with a known equalizer in the receiver node.
The local oscillator typically includes a laser, which outputs continuous wave (CW) light, that effectively serves as a reference to which the phase, for example, of the incoming signal is compared, such that the data carried by the incoming optical signals can be identified. Light output from the local oscillator, however, may vary in phase. Such additional impairments or phase variations of the local oscillator light are related to the linewidth of the local oscillator laser and other noise sources.
As noted above, local oscillator light is combined with the received optical signal light in the optical hybrid circuit. Accordingly, the digital samples output from the ADC may reflect distortions associated with the local oscillator impairments. The phase variations of the local oscillator light, however, are different from the variations and other impairments experienced by the received optical signals. As a result, a CD compensating equalizer typically does not compensate or offset such distortions associated with the local oscillator light. Moreover, the CD compensating equalizer may introduce further noise in the data output from the receiver node.
Accordingly, there is a need for a receiver node that can minimize impairments associated with a local oscillator.